In many processes the reactants or the fluids are mixed before entering a heat-exchanging zone and the mixture is then brought to the desired temperature in the heat-exchanging zone. In some processes the heat is transferred from the flow of fluids by heat exchangers connected in series and before each heat exchanger one or more reactants or fluids are added in batch wise portions. According to another practice the reaction flow is circulated in order to restrict any temperature rise. A further solution, which is used, is to connect one or more mixing vessels in series in a so-called cascade of mixing vessels.
All of the above-mentioned solutions have drawbacks because, among other things, they are complicated and expensive.